Great question!Let me share my experience.I am 25 and have been working for 2.5 years.I started contributing to my 401(k) immediately.However I was a beginner at this, not and enlightened Fool. So I put my money in a mix of funds, some holding bonds , some holding stocks, and some holding both.I basically put money in every fund available to me.I was getting good returns out of the stock funds, but poor returns out of the others.After reading the Fool and learning more about investments, I changed the funds I held.I only have 4 funds in my 401(k) now. All of them are stock funds. One is an S&P 500 Index fund (allocated at 45%). The other 3 funds all have low management fees and good returns.I am not too aggressive with the funds that I chose. This mix seems to be doing very well for me considering the general downturn in the market. While the returns are not great, I am not losing money either.I do not plan on holding individual stocks in my 401(k). The reason is simple. The money that I put into my 401(k) is money I can't touch for 40+ years. If I were to invest in stocks, I would be a fool to not re-evaluate these investments every quarter. With mutual funds, I look for funds that have a reasonable investing strategy.With mutual funds I feel I only need to re-evaluate my holdings if the management or investing strategy of the fund changes. Much less maintenance involved.So far this has all worked out well for me. I plan on sticking to the above philosophy and contributing as much as I can to my 401(k). It is amazing how fast the money builds up!Ultimately how aggressive you are depends on you own personal preference. Good Luck!